A variety of magnetometers for weak field detection are known in the art. One such is taught in U.S. Pat. No. 4,851,775 by Kim et al. The Kim magnetometer is an oscillator whose frequency of oscillation changes as a function of applied magnetic field, and because of this the sampling rate is limited for fast moving magnetic fields. In using the Kim magnetometer, in order to obtain adequate resolution for measuring frequency shifts of an oscillating signal centered around, for example, 50 Khz, one must count up to 300 cycles of the signal waveform against a 5 Mhz gating clock in order to obtain 150 nT field resolutions. This limits the ability to sample field changes to only about a 50 Hz rate. Furthermore, since multiple cycles must be measured, the sensor circuit becomes an inherent low-pass filter. Although this limitation might be desirable for rejecting unwanted high frequency noise components, such as 60 Hz powerline noise, there is no good technique which can be used with this frequency detection scheme to purposely measure higher frequency events.
The frequency approach in magnetometer technology is also inherently noisy, as a slow comparator is required in order to reject spurious noise from sources such as a microprocessor clock on a power supply in a digitized system. Since a slow comparator moves slowly through the transition range, it is susceptible to frequency modulation (FM) caused by what is known in the art as the Barkhausen effect. This in turn creates noise. This FM limits the bottom of the noise floor to about 15 nT. Power consumption also is relatively high for frequency-based magnetometer systems since such systems must operate on both halves of each oscillator cycle. Typically, in instruments employing this technique, each positive-going cycle must charge up to the positive hysteresis point of a Schmitt trigger as set by a feedback resistor, while each negative cycle must charge down to the negative hysteresis point as well. When the area under the curve is integrated, it can be seen that more power is expended than if only one half of a cycle were used.
What is clearly needed is an apparatus and method for measuring weak magnetic fields, such as the magnetic field of the Earth, which is accurate, relatively inexpensive to manufacture, less noisy than conventional apparatus and techniques, and not inherently bandwidth limited.